An enzyme/prodrug strategy incorporating multi-modal real-time imaging of prodrug enzyme delivery and clearance was designed to optimally time prodrug administration and limit systemic drug toxicity. Tracking fluorescently labeled bacterial cytosine deaminase (red) uptake by liver Kupffer cells (green), where co-localization is in yellow, exemplifies one imaging modality of this multimodal imaging approach. For further details, please see Li and colleagues on page 515 in this issue.

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